1. Field of the Invention
The present invention relates to an iodine-type polarizing plate comprising a uniaxial stretched polyvinyl alcohol resin film with iodine adsorbed thereto in an oriented state and a protection film bonded to at least one surface of the resin film.
2. Description of the Related Art
A polarizing film made of a polyvinyl alcohol resin film with a dichromatic substance adsorbed thereto in an oriented state is used as one of members of a liquid crystal display device, normally in the form of a polarizing plate with a protection film bonded to at least one surface of the polarizing film. The conventional polarizing plate however has a problem as follows. When the polarizing plate is used for a liquid crystal display device, in particular, for a reflective liquid crystal display device, a screen of a liquid crystal display appears yellowish in the case of white display. To solve this problem, Japanese Laid-Open Patent Publication No. 8-304624, for example, discloses that coloring of the screen in the case of white display can be suppressed by changing the shape of a transmission spectrum of the polarizing plate.
However, the polarizing plate disclosed in the above publication is low in degree of polarization compared with generally used polarizing plates, and exhibits bluish hue of the screen in the case of black display. Therefore, a liquid crystal display screen using such a polarizing plate has problems of being low in contrast and having bluish hue in the case of black display. In recent years, various types of color reflective liquid crystal display devices have been realized at rapid paces. Under these circumstances, it is demanded to develop a polarizing plate that dose not provide yellowish hue of the screen in the case of white display and bluish hue of the screen in the case of black display, and enables high-contrast display.
Hereinafter, a hue, which is not yellowish and is white as it is called, is referred to neutral white, and a hue, which is not bluish and is black as it is called, is referred to neutral black.
The present inventors have earnestly studied in order to develop a polarizing plate enabling neutral white in the case of white display and neutral black in the case of black display, bright white display, and high-contrast display, and found that an iodine-type polarizing plate satisfying condition as described herein could provide the intended performance. That is, the iodine-type polarizing plate, which is a polarizing plate using iodine as the dichromatic substance, have a single transmittance of about 42.5% or more, a degree of polarization of about 99% or more, coefficients, calculated from a hue in the parallel state and a hue in the perpendicular state in L*a*b* color system, are within a range as described herein. The present invention is based on these findings.
The hue in the parallel state as used herein is a hue when two polarizing plates layered one upon the other so that their absorption axes match with each other are irradiated with natural light (non-polarized light). This hue is hereinafter referred to a xe2x80x9cparallel huexe2x80x9d. The hue in the perpendicular state as used herein is a hue obtained when two polarizing plates layered one upon the other so that their absorption axes perpendicular each other at right angles are irradiated with natural light (non-polarized light). This hue is hereinafter referred to a xe2x80x9cperpendicular huexe2x80x9d
An object of the present invention is to provide a polarizing plate that enables neutral white in the case of white display and neutral black in the case of black display, bright white display, and high-contrast display.
The present invention provides an iodine-type polarizing plate comprising a uniaxial stretched polyvinyl alcohol resin film with iodine adsorbed thereto in an oriented state and a protection film bonded to at least one surface of the resin film, wherein the polarizing plate has a single transmittance of about 42.5% or more, a degree of polarization of about 99% or more, and a neutral coefficient Np of 0 to about 4 which is calculated from the following expression (I), and the polarizing plate satisfies the relationship represented by the following expression (II);
Np=|{square root over ((Pa*)2+(Pb*)2)}xe2x88x92{square root over ((Qa*)2+(Qb*)2)}|xe2x80x83xe2x80x83(II)
wherein Pa*, Pb*, Qa*, and Qb* respectively denote a* and b* in a parallel hue and a* and b* in a perpendicular hue in L*a*b* color system;
{square root over ((Pa*)2+(Pb*)2)} less than 4.5xe2x80x83xe2x80x83(II)